MTFCA Forum

I am trying to repair a few NH carburetors.
As they were very rusty, I soak them for 24 hours in a rust remover.
After having cleaned up everything some threads are wide and need a repair.
The thread size I am looking for are:
The thread of the spray needle;
The thread of the float valve;
The thread of the drain valve in the carburetor housing;

For the spray needle and the float valve I found 1/4" x 32.
For the drain valve I found 9/16" x 24.
As I am not used to this imperial threads so before ordering parts I am willing to be sure about the things I found.

Are these right.
Can these threads be repaired by using a heli coil (here they call it a V-coil)?

I cleaned up the float valve and the spray needle thread in the housing with a 1/4" x 32 tap and both of the thread are wide and the valve and the needle are turning free in the housing.
Thanks
Andre
Belgium

My NH carb was so rusty that I had to replace parts.
1/4" UNC is 20 threads per inch and UNF is 28. So, 32 is surprising as it is non-standard but Henry was also OK with using #14 which is close to 1/4".
I can check mine the next time I am at the garage if no one else answers.

Vernon,

On the Tap I used to clean up the thread I found "UNEF 1/4" - 32 HSSG".
The 32 match with the threads only the diameter of the needle and the valve are closer to 6mm as to 1/4".
I think you could be right for the 14 x 32 original thread.
I will try to make the repair the thread by using a 6mm V-coil and will adapt the thread on the valve and the needle to fit the 6mm V-coil.

For the drain valve 9/16 x 24, I didn't fined a V-coil, may be out there you have afitting Heli-coil.

Thanks
Andre
Belgium

I hope this will help you... Rich C.

1/4-32 is a very common thread size in carburetors.
Not in NH but 5/16-27, 3/8-27 , 7/16-27 are common.
3/8 are usually standard 16 or 24 but many are also 3/8-20.
The fuel fitting is standard 1/2-18 except for some early 1/2-20.
The bowl fitting on early Kingston is 17/32-18.
The bottom thread on NHs is 9/16 -24, which can be a hard and expensive size to find.
For some reason ebay usually has some used ones for cheap.

If you really want to save an NH body with bad threads you can tap it 5/8-24, cut the male thread down and solder a bushing on it threaded 5/8 -24 and enlarge the hole in the bowl.
It takes longer to write about it than to do it.

I have over 100 non standard size taps, some I made from drill steel and hardened but there are places that have taps from 080 64 to 3 inch.

A 1/4 - 32 tap and die set is a good investment.

Andre, if you have a lathe it is easy to do the bushing or ream the hole and tap it 11/16-24, lock tight a bushing with female 9/16-24 threads in and use the original drain/bowl fitting.

Thanks Stan, thanks all,

Just a question: What mean "tpi" behind the thread size on the sheet?

With the sugestions of Stan I will work something out and will come back with the way I did it.

Andre
Belgium

Andre', TPI = Thread per inch . Aantal gangen per inch .

Toon

The Imperial thread system is slightly different in nomenclature compared to the metric. Metric sizes are in MM with the thread pitch following. 14 1.25 or 1.50 etc. Imperial is the size followed by the TPI - threads per inch. 1/4 x 20, 1/4 x 28 etc. There are two "standard" pitches or TPI for each size, noted as coarse and fine. Fine is often noted as SAE. Then there are all sorts of "Special" thread sizes in each diameter. 1/4 x 32, 1/4 x 40 etc. Sizes smaller than 1/4 are noted either as fractional sizes - 3/16 x 24 - or single number sizes 10 - 24, 10 - 32 etc. It can really get confusing.